1. Field of the Invention
This invention relates to a process for polymerizing propylene. More particularly, it relates to a process for polymerizing propylene by the use of a catalytic system composed of a highly active, solid titanium trichloride-based catalytic complex and a specific organic aluminum compound.
2. Description of the Prior Art
It is well known to use a solid titanium trichloride-based catalytic complex as a catalyst and an organic aluminum compound as a cocatalyst for the stereospecific polymerization of propylene. It is also known that the organic aluminum compound gives different influences on activity for polymerization and stereospecificity depending on the type thereof. In known processes of polymerizing propylene using an organic aluminum compound as a cocatalyst, however, a cocatalyst represented, for example, by the general formula, AlR.sub.2 X (in which R represents an alkyl group and X represents a halogen atom) contributes to impart high stereospecificity but is low in polymerization activity, so that the amount of polymer formed per unit amount of catalyst (hereinafter referred to as catalytic efficiency) is low, thus requiring an additional step of removing the catalyst from the polymer product. On the other hand, a cocatalyst represented by the general formula, AlR.sub.3 (in which R represents an alkyl group) gives high polymerization activity and is thus high in catalytic efficiency. However, the cocatalyst of this type results in a polymer of very low stereospecificity, requiring a step of removing amorphous polymer from the polymer product. Thus, both types of the aluminum compounds are disadvantageous.
In order to improve the polymerization activity, when an organic aluminum compound expressed by the general formula, AlR.sub.2 X, particularly, diethylaluminum monochloride now widely used industrially, is used as a cocatalyst, it is known to add to the above organic aluminum compound small amounts of a compound expressed by the general formula, AlR.sub.3, e.g. triethylaluminum, triisobutylaluminum or the like in such a manner that the ratio of R to Al is in the range of above 2:1. In this case, though the polymerization activity is improved as will be clearly seen from Comparative Examples shown hereinafter, the stereospecificity is lowered to a considerably extent. Thus, this catalytic system is considered disadvantageous as an industrial catalyst in view of its poor practicability. From the above, it has been revealed that when diethylaluminum monochloride is used as a cocatalyst, its purity is an important factor for retaining the stereospecificity advantageously developed from the monochloride compound. In other words, it has been found that it is important to keep the ratio of an ethyl group to Al at 2:1.
An intensive study has now been made of a cocatalyst expressed by the general formula, AlR.sub.2 X. As a result, it has been found that a catalytic system using a compound expressed by the general formula, AlR.sub.n Cl.sub.3-n with the R and n being specifically defined is high in polymerization activity and ensures high stereospecificity.